These in vivo studies are designed to (1) characterize the estrogen receptor system in the hypothalamus, pituitary and uterus of the rat as a function of age and reproductive state. Qualitative properties of the different forms of estrogen receptor, their compartmentalization, and the process of intranuclear transformation in the biology of estrogen action, will be assessed by sucrose density gradient ultracentrifugation. Quantitation of the total tissue estrogen receptor content will be effected by use of the cytoplasmic and nuclear exchange assays. The physiologic process of developing pituitary sensitivity to a hypothalamic influence as mediated by estrogen milieus will be studied in both the pre and postpubertal state. The relationship between the total pituitary estrogen receptor content, its qualitative aspects, and the responsivity of the gland to synthetic or native hypothalamic luteinizing hormone releasing factor (LRF) will be determined by quantifying serum LH levels. The relevance of the proposed research projects lies in advancing our knowledge and understanding of the biology of estrogen action, in vivo, in dissimilar estrogen target tissues.